The present invention generally relates to valve gate systems wherein a reciprocating valve pin opens and closes a melt passageway in injection molding applications. More particularly, this invention relates to valve pin assembly and a method for adjusting valve pin position to ensure proper opening and closing of the melt passageway during injection molding operations.
Valve gated injection molding systems are known in which pistons are actuated pneumatically or hydraulically to reciprocate the valve member. In general, such systems require that the components, in particular the valve pin and its associated piston, be made to very close tolerances to allow for thermal expansion so that, when at operating temperatures, the front end of the valve pin seats precisely in the gate to close it. If the valve pin does not extend far enough into the gate, it will not seat tightly when in the closed position, leading to a poor gate vestige. If the valve pin extends too far toward the gate, the tip of the pin will repeatedly impact the gate on closing, thus causing undue wear of both the valve pin and the gate and, potentially, fracture of the mold around the gate.
Typically, final adjustment of the positioning of the valve pin is a complicated and time-consuming procedure. First, the hot runner system in which the valve pins are to be used is fitted to the mold plate(s) without the upper clamp plate and valve pins in place. The valve pins, which have been cut to a calculated length, are inserted into the assembly. A depth gauge is used to measure the position of the valve pin head at room temperature from a reference point, usually the back surface of the mold plate. The assembly is then heated to the processing temperature and held at that temperature for a period of time sufficient for all the hot runner components, including the valve pins, to be uniformly heated. The depth gauge is then used to measure the position of the valve pin head at operating temperature (the valve pin head's new position), and the amount of the adjustment of each valve pin is calculated based upon the differences of the two measurements. Specifically, if the valve pin is too long, it must be removed and shortened; if the valve pin is too short, the piston that carries the valve pin is removed and its bottom is ground to shorten the piston. This measurement and shortening/lengthening process is repeated for each valve pin in the system. It is, obviously, an exacting and time consuming process.
Accordingly, it is an object of the present invention to provide an improved valve pin assembly and method for adjusting the valve pin that facilitates the quick and precise positioning of the tip of the valve pin with respect to the mold gate.